Communications and surveillance jammers are common in almost every channel of communication. Radio jamming, radar jamming and deception, and mobile phone jamming, may be among the most encountered forms of jamming interferences. Many communications and surveillance systems may need to be equipped with anti jamming equipment or devices to prevent or resist being jammed by interferers.
There are several existing techniques for performing interference or jammer cancellation on radio frequency signals. These techniques may fall into six general categories: (1) fixed or adaptive signal modulation; (2) fixed or adaptive forward error correction (FEC) or coding; (3) fixed spatial antenna patterns; (4) adaptive spatial antenna patterns; (5) spectrum spreading (e.g., direct sequence spread spectrum (DSSS), frequency hopping, or both); and (6) temporal cancellation. Often, combinations of these techniques may be used simultaneously. An exemplar system might include a combination of categories (1), (2), (3), (5), and (6) simultaneously. Such a system may include a range of adaptive modulations and coding. The more robust modulations and FEC coding (e.g., those that require lower energy-per-bit-to-noise-density ratio (Eb/No) may be used when jamming is detected to increase communication robustness. More bandwidth efficient modulations and coding would be used when no jamming is detected. This system may use fixed sector antenna patterns (3) to reduce interference or jamming that originates outside of its intended sector. Finally, this system might also use either a direct sequence spread spectrum (DSSS) modulation or frequency-hop its signal to reduce the effect of jammers.
One method of jammer excision involves a beamforming technique that combines a jammer plus Signal-of-Interest (SOI) autocorrelation measurement with a constraint vector (aka constraint vector). The system of the subject technology can null all received energy except in the direction of the constraint vector. If the constraint vector is pointing at the SOI then the jammer is nulled and the SOI is copied. In this technique, to constrain the beam-former from nulling the SOI, a constraint aperture vector may be employed. The constraint aperture vector may be based on measuring correlation against pilot signals embedded in the SOI. In order to obtain an accurate estimate of the constraint aperture vector, the system may have to perform correlation for a very long period of time. This technique can be slow and impractical in the real world because during this very long correlation interval (e.g., minutes), it may be difficult to maintain synchronization using the pilot symbols under heavy interference. SOI motion during this long interval can be less predictable because it may be non-linear with continuous change in direction and velocity. Accordingly, SOI experiences phase rotation due to motion induced Doppler Effect that can result in unpredictable synchronization drift, which can prevent obtaining a clean constant vector. Therefore, the need exist for a faster and more practical approach for jammer excision.